I would probably get a sonotube from the hardware store or a "big box" home improvement chain. They are cardboard tubes for concrete but you can use it as a form for you cactus. You could even cut holes in the sides for the cactus arms, just make some smaller tubes out of chicken wire to fit in. Then al you need is a cap for the top and you could cover it in papier mache and paint. Good luck. We'd all love to see pics!

I'm guessing you want a saguaro cactus. Stack oatmeal cartons or even metal cans and tape them at the seams to make a tower as tall as you want it. Place your tower so that there is an open end at the top. Blow up a balloon so that it will fit snugly in the opening at the top. Put the balloon in so the top will be nicely rounded. You now have a straight upright cactus. Go to a hardware store and purchase some flexible hosing. If your cactus is made of Pringles cans you want hosing like they use to clean swimming pools. If the cactus is made of coffee cans, you want hosing like you would use on your dryer. In other words, you want it to be size appropriate. Bend a clothes hanger wire in half and then bend it to the approximate shape you want the arms to be. Next tape the wire inside the hose so that there is about an inch or more of wire sticking out where it will connect to the tower. Bend that inch of wire outward at a 90 degree angle so that it can be taped to the tower when the time comes. Again, blow up a balloon and tuck it into the top end of the arms. Now tape your arms in place OR make a hole in the tower large enough to fit the arm into. Depending on the size of the catus and the weight of the arms, fitting the arms into a hole is the better choice. Cover the whole thing with paper mache and paint it green. The arms should be fairly sturdy once the paper mache has dried. I made several with Pringles cans to use as center pieces on tables and they looked cute. One thing though!!! I was raised in Arizona and can tell you that Arizona is the ONLY state that saguaro cactus is native. It drives me crazy when I see businesses in Texas using them as a symbol of their state. I'm not sure how many cactus you want to make, but if you Google 'pictures of cactus' you should see lots of different kinds and some are easier to make than others. Barrel cactus has no arms, so it should be super easy. Good luck and have fun!!

You've gotten some great suggestions for making cactus. My suggestion isn't so much making them as painting them. Paint the bottom of a tennis shoe that has grids on them and they make cute cactuses. Just keep adding to the tennis shoe soles going up and out. They are really cute looking and do look a lot like cactus. You might use one of the shape ideas above and then cover in paper which you have painted with the tennis shoe stencil. I would love to see this when you are done. (I used to make these with my students when I taught 2nd grade. We always had tennis shoes in the lost and found to use.) Good luck!!